This invention relates to a system for controlling transmission power in an earth or terrestrial station of a satellite communication network.
In satellite communication, rain or mist gives rise to a considerably severe attenuation particularly when higher frequencies, as in the so-called K band, are used. A suitable countermeasure is indispensable for the attenuation.
For the attenuation caused to a down link signal radiated by a satellite to an earth station, receiving installation of the earth station is adequately given a margin which is sufficient to compensate for the attenuation. It is, however, objectionable to make transmitting installation of the earth station transmit an up link signal with a high transmission power to preliminarily compensate for the attenuation which may be caused to the up link signal. This is because a transponder on board the satellite must make the satellite radiate down link signals to destination stations with an accordingly high effective isotropically radiated power (EIRP) to waste electric power on the satellite when it is clear along a path or route for the up link signal. It has therefore been proposed to control the transmission power of the up link signal in consideration of the up link rain or mist attenuation. Such a proposal is generally called control of the earth station transmission power.
In an article contributed in the Japanese language by Syun'itiro EGAMI et al to "Summaries of Papers to Be Read at 1978 General Meeting of the Institute of Electronics and Communication Engineers of Japan," pages 8-293 and 294, Paper No. S10-11 entitled `K Band Mobile Communication System` (in translation), a proposal is described for a satellite communication network comprising a mobile station, which may be carried by a helicopter, and a fixed earth station. According to Egami et al, baseband pilot signals are exchanged between the mobile and the fixed stations through an up link channel, the satellite, and a down link channel. The pilot signals are used in measuring the signal-to-noise ratios of the mobile-to-fixed and the reverse paths. The proposal is restricted to application to only two earth stations. When one of the two earth stations is taken into consideration, a counterpart earth station is indispensable. Furthermore, a specific frequency is additionally necessary in each of the up and the down link frequency bands for use in exclusively transmitting each of the baseband pilot signals.
In another article contributed in the Japanese language by Minoru Yamamoto et al to "Summaries of Papers to Be Read at 1978 Meeting of Opto-Electromagnetic Wave Section of the Institute of Electronics and Communication Engineers of Japan," page 182, Paper No. 182 entitled `On Compensation for Rain Attenuation in the Up Link of a BS Main Station` (in translation), two schemes are proposed. According to a first scheme, the power with which an up link signal reaches the satellite is measured on the satellite and then sent back to the main station on a telemeter signal. The satellite must therefore be equipped with specific installation for use in exclusively carrying out the control. Moreover, it is inconvenient to apply the scheme to other earth stations.
According to a second scheme of the Yamamoto et al article, the up link rain attenuation is estimated from a level reduction caused by rain in a two-way signal which is sent from the main station as an up link signal to a satellite and then sent back thereto as a down link signal from the satellite. The estimation is based on a correlation between the rain attenuation for the up link frequency band and that for the down link frequency band. The main station must therefore comprise receiving installation for the return signal sent back thereto from the satellite. Furthermore, an estimation error is inevitable because the correlation factor is liable to fluctuate.